Convective-scale variations in the inner core rainbands of tropical cyclones

The inner core of a tropical cyclone exhibits a vast array of convection outside of the eyewall, which is usually organized into multiple rainbands. In order to determine the role that this convection has in modifying the storm intensity, we must understand the variations in the convective-scale structures that the convection exhibits. During the 2005 Hurricane Rainband and Intensity Change Experiment (RAINEX), the Electra Doppler radar (ELDORA) obtained high-resolution observations of a large sample of inner core convection in Hurricane Rita. Using these observations, we have examined and characterized the radial and azimuthal variability of the convective-scale reflectivity and kinematic structures in order to seek the underlying dynamics that connect the various precipitation features and also drive their variability.

The azimuthal variations of the convection follow results from past studies that connect the orientation of rainband convection to the environmental vertical wind shear. At larger (smaller) radii, the downshear right (left) quadrant contained the most convective cells, and stratiform precipitation was most prevalent in the quadrant just downwind from this sector.

All convective cells had a common circulation pattern of deep radial inflow, upward motion through the reflectivity tower, and subsequent radial outflow at higher levels; however, details of the kinematic structure varied with the radial distance of the convective cell from the storm center. Cells at smaller radii contained a tangential jet in the low levels that was driven mostly by inward radial advection of tangential momentum, while cells at larger radii contained a jet in the either the low or middle levels (or both) driven by inward radial advection and vertical advection of tangential momentum. The subsequent outflow layer was at a higher (lower) altitude for the outer (inner) convective cells.

Radial variations in the convective cells can be attributed to differences in CAPE and tangential wind speed. Larger CAPE at larger radii deepens the convection and enhances vertical advection of momentum, which can lead to the generation of mid-level tangential jets. Larger tangential wind speeds at smaller radii cause convectively-generated jets to have a stronger supergradient response in a slowly changing vortex-scale pressure distribution. The stronger supergradient response causes rising air to be ejected outward at a faster rate and thus at a lower height. This leads to a shallower inflow layer that constrains the tangential jet to lower altitudes. The stronger supergradient force and lower CAPE at smaller radii can lead to greater amounts and longer lifetimes of active convection. A lower jet enhances the WISHE feedback, and a lower outflow layer enhances convergence with the inflow of higher moist static energy air. The enhanced amount and sustenance of active convection can affect the storm dynamics by generating more potential vorticity that feeds into the storm core or by increasing the likelihood of secondary eyewall formation.